Register a core ask_user custom tool that posts a question set to the user's
browser and terminates the run instead of awaiting an answer. The tool is thin:
it shapes its TypeBox params into domain questions, lets PendingAskStore own
validation, and reports a superseded unanswered ask back to the model.
Gated by the askUser config key, threaded through PiSessionServiceDependencies
and sessiond. Unlike the delegation tools, ask_user is available to tracked
children too: the questions reach the user of the asking session.
